Publication in Astronomy & Astrophysics: »Height variation of the cutoff frequency in a sunspot umbra«

In a joint observing campaign at GREGOR in June 2017, scientists from three different institutes (IAC, KIS, and AIP) observed the oscillations in the umbra of a sunspot. Two instruments, GRIS and GFPI, were used to acquire spectropolarimetric and spectroscopic data, respectively, at different wavelengths. With this setup, it was possible to infer the velocities across several heights of the solar atmosphere.

T. Felipe (IAC), C. Kuckein (AIP), and I. Thaler (KIS, Racah Institute of Physics) found evidences of the variation of the cutoff frequency with height in the umbra of the sunspot. The acoustic cutoff frequency is a local quantity which depends on the height of the atmosphere. It separates the high-frequency propagating waves from the low-frequency transient waves. The height dependence of the cutoff frequency has implications for helioseismology as well as for a better understanding of the wave propagation on the Sun.

The results appear in the refereed journal Astronomy & Astrophysics and are also available online on arXiv.

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